CAPE CANAVERAL, Florida — A fuel leak and then an engine problem during final liftoff preparations led NASA to scrub the launch of its mighty new moon rocket Monday morning on a shakedown flight with three test dummies aboard.

The next launch attempt will not take place until Friday at the earliest.

As precious minutes ticked away, NASA repeatedly stopped and started the fueling of the Space Launch System rocket with nearly 1 million gallons of super-cold hydrogen and oxygen because of a leak of highly explosive hydrogen in the same place that saw seepage during a dress rehearsal back in the spring.

Then, NASA ran into new trouble when it was unable to properly chill one of the rocket’s four main engines, officials said. Engineers continued working to gather data and pinpoint the source of the problem after the launch postponement was announced.

The rocket was set to lift off on a mission to put a crew capsule into orbit around the moon. The launch represents a milestone in America’s quest to put astronauts back on the lunar surface for the first time since the Apollo program ended 50 years ago.

The 322-foot spaceship is the most powerful rocket ever built by NASA, out-muscling even the Saturn V that took the Apollo astronauts to the moon.

As for when NASA might make another launch attempt, launch commentator Derrol Nail said the problem was still being analyzed, and “we must wait to see what shakes out from their test data.”

No astronauts were inside the rocket’s Orion capsule. Instead, the test dummies, fitted with sensors to measure vibration, cosmic radiation and other conditions, were strapped in for the six-week mission, scheduled to end with the capsule’s splashdown in the Pacific in October.

Even though no one was on board, thousands of people jammed the coast to see the rocket soar. Vice President Kamala Harris was expected among the VIPs.

The launch, when it happens, will be the first flight in NASA’s 21st-century moon-exploration program, named Artemis after Apollo’s mythological twin sister.

The problems seen Monday were reminiscent of NASA’s space shuttle era, when hydrogen fuel leaks disrupted countdowns and delayed a string of launches back in 1990.